The present invention relates to a method for the preparation of globular particles of a rare earth oxide. More particularly, the invention relates to a method for the preparation of rare earth oxide particles having an outstandingly globular particle configuration and suitable as a base material for the preparation of various kinds of phosphors.
While rare earth oxides in the form of a powder are widely used as a base material of various kinds of phosphors in fluorescent lamps, cathode ray tubes and the like, it is known that the particles of a phosphor powder should desirably have a particle configuration as close to spherical as possible in order that the phosphor composition prepared therefrom may exhibit good workability in the coating works and a high efficiency of luminescence. In view of this requirement, various attempts and proposals have been made heretofore to provide a method for the preparation of a rare earth oxide powder having a spherical or globular particle configuration because it is empirically known that, when a rare earth oxide powder having a globular particle configuration is used as the base material of a phosphor, which is prepared by mixing the rare earth oxide powder with other starting materials and a flux followed by calcination of the mixture, the particle configuration of the phosphor powder can also be globular.
For example, Japanese Patent Kokai 3-23214 discloses a method according to which globular particles of a rare earth oxide can be obtained by granulation of very fine particles of the oxide. This method of granulation, however, is not applicable to the preparation of a rare earth oxide powder suitable as the base material of a phosphor because the particles obtained by this method is too coarse usually having a diameter of 20 .mu.m or even larger to exceed the suitable range of particle diameter as the base material of a phosphor.
Further, Japanese Patent Kokai 3-27117 and 3-27118 each disclose a method for the preparation of globular particles of a rare earth oxide which can be obtained by the calcination of a specifically prepared rare earth oxalate powder having a globular particle configuration. A problem in these methods is that the precipitation reaction of the rare earth oxalate must be conducted at a low temperature and dehydration and drying of the oxalate powder must be performed under specifically controlled conditions so that the productivity of the process is not high enough.